The TOS movies were used to revive what had been a canceled franchise, and they expanded on what little we had seen in the TV series. Plus, TOS only had 3 central characters so it was easy to keep that for the movies.

Also, the TOS movies were where these character finally started to show growth as characters that they never got to do in the TV series.

The TNG movies, however, couldn't handle the ensemble cast so well and TNG had pretty much run its course by then anyways. There was little purpose to the TNG movies to begin with (and Paramount made them on the cheap, too).

Frankly, the best ideas for TNG movies had already been done as TV episodes: Yesterday's Enterprise, Best of Both Worlds, heck even Descent.